This invention relates to a unique method of producing thin films of refractory materials (characterized as those materials that at temperatures greater than 1000.degree. C. do not decompose and retain their strength properties) that can be used as bi-directional reinforcing materials in lightweight-high strength composite materials. Current art relates to high strength-high modulus filaments or fibers produced by vapor deposition or pyrolysis which are used as uni-directional reinforcements for composite materials. Typical of these are boron filaments produced by vapor deposition of boron on fine tungsten wire, or carbon and graphite filaments produced by controlled pyrolysis of rayon or PAN fibers. These fibers, though strong and stiff, lend only uni-directional reinforcement; and, thus, present problems related to interlaminar shear and complex filament winding techniques.
This invention overcomes these disadvantages by producing the reinforcing material in the form of a film instead of filament, which film can be used in laminated structures over a wide size range, and by producing the film on a liquid substrate provides a large, smooth, flat surfce area from which the film can be continuously separated. The combination of the imperfection and stress-free liquid surface and a substrate surface temperature conducive to crystal mobility provides the means for producing a thin film whose properties approach that of currently available fiber or filament uni-directional reinforcements. If film structure is not adequate, due to deposition temperatures set by liquid substrate reactivity or vapor pressure, further annealing or stress annealing can be accomplished after the film is separated from the substrate.